Omeprazole is a generic drug used to treat certain conditions where there is too much acid in the stomach. It is used to treat gastric and duodenal ulcers, erosive esophagitis, and gastroesophageal reflux disease (GERD). GERD is a condition where the acid in the stomach washes back up into the esophagus. Omeprazole is a proton pump inhibitor (PPI). It works by decreasing the amount of acid produced by the stomach.